The use of plastic sheet material such as polyvinyl chloride and the polyolefin plastics such as the polyethylenes and the polypropylenes has been recently approved by the United States Food and Drug Administration for the manufacture of transparent bottles and other containers for food products. As indicated in the Apr. 5, 1969 edition of "Chemical Week," on page 47, the bottling industry is forecasting resin consumption for this purpose to grow to more than 150 million pounds within the next five years and changes in the use of molded polyvinyl chloride food containers indicate this to be realistic.
The major resins which are coming into widespread use for food packaging are polyethylene, polyvinyl chloride, polystyrene, acrylic resins alone or blended with ABS rubbers and polypropylene since in the bottling industry alone, the use of these resins has considerable economy, increased transparency, ease of handling economics such as lower shipping cost and less breakage and product protection in that they act as a very effective barrier to oxygen and thus assist in the preservation of the food content. However, at the present time, there are no acceptable organic dyestuffs which are approved for use in contact with foods and beverages, medicinal products or cosmetics and at the same time are compatible with any of the aforesaid principal resinous molding compositions. In the instance where it has been sought to introduce color into such molded containers, it has heretofore been necessary to mount the dye on a carrier such as aluminum hydrate. In addition to the obvious loss of transparency of the plastic container due to the inclusion of the opaque pigment particles, there has been encountered a problem which is particularly acute with those resins, such as polyethylene and polypropylene where it is necessary to mold the resinous compositions with temperatures on the order of 350.degree. F. Under these molding conditions, the aluminum hydrate carrier for the pigment thermally decomposes and releases its bound water which thereupon forms minute droplets which ultimately join to form bubbles and blisters in the final molded article.
In still other instances, where it is sought to blend presently acceptable food grade organic dyestuffs with a resinous polymer solution, the dyestuff particles being insoluble in the resin solution, do not uniformly disperse themselves through the polymer liquid and result in a final molded product which has a granular or grainy appearance which is ineffective as a means of color distribution in the plastic molded article. This is the result of the insolubility of certified F.D. and C colors in organic solvent systems such as methyl ethyl ketone or acetone or liquids having a higher hydrocarbon moiety. Under these conditions these colors will persist as undissolved discrete particles having little or no tinctorial power. Even where high sheer mixing techniques have been employed in an effort to increase the particular dispersion of the insoluble organic dyestuff in the resinous base, it has not heretofore been practical to maintain a uniform color distribution during the course of transport and usage of the molding composition up to the point where it is completely molded into a shaped package, bottle or container. As a consequence, therefore, the use of attractive coloration which is also cleared as safe from potentially harmful side effects has been limited to the use of titanium dioxide pigment which, of course, gives color but prohibits transparency and lake pigment which must be employed on a carrier substance which also obviates transparency in the article tinted or colored.
It is an object of the present invention to overcome all of these disadvantages in the art of coloring plastic food, drug and cosmetic containers and to present to the art a resinous molding composition which is not only acceptable as safe for contact with consummable items but which resin is also colored in various shades and transparent to any degree desired.
It is a further object of the invention to describe a new and unexpected technique which comprises the compatible blending and uniform solution of food grade color additives which are normally considered to be incompatible with resinous solutions in all proportions. These and several other objects of the invention will become clear upon further consideration of the detailed description of the invention set forth in the following general description and several selected and preferred modes of its practice.